1. Field of the Invention
This invention relates to electric power measuring apparatus. More particularly, this invention relates to analog-to-digital (A-to-D) converters useful in systems for measuring the consumption of electric power, for example, power delivered to a home by an electric utility.
2. Description of the Prior Art
Electric power delivered to homes or commercial buildings has for many years commonly been measured by means of a well-known electro-mechanical type of integrating meter incorporating a rotating disc. Such meters have found widespread and long time use because they not only are highly accurate but also are relatively inexpensive to manufacture. However, they have an important disadvantage in not being capable of readily interfacing with and/or taking advantage of modern digital data systems. Consequently, there has developed a need for an electric power meter which is essentially electronic in nature, particularly in being able to produce digital output signals accurately reflecting power consumption.
It has been proposed that the magnitudes of the voltage and current supplied to a customer be sampled periodically at a relatively high frequency, and converted to respective digital signals as by the use of known A-to-D converters. These digital signals would then be multiplied in conventional fashion and integrated over time to provide total power consumption. This approach is theoretically workable, but because of the stringent accuracy requirements applicable to electric power measurement, the employment of A-to-D converters using known technology would lead to undesirably high manufacturing costs.
In more detail, the accuracy specifications for electric power meters typically require that a measurement at full-scale power be accurate to 1%, and that a measurement at 1% of full-scale power also be accurate to 1%. Achieving an accuracy of 1% at 100% of full scale is well within the capability of present day A-to-D converters. However, for such equipment also to provide an assured accuracy of 1% at the other end of the range, that is, at only 1% of full scale, is a very severe requirement. It has, for example, been calculated that achieving such an accuracy would require a very high quality A-to-D converter with a 16-bit resolution. Although such devices can be made, they are intricate in design and costly to make.
The accuracy problem is particularly acute with respect to making measurements of the electric current component of the power being consumed, because the current being drawn can vary anywhere from zero to full-scale value. On the other hand, measurement of the voltage component is relatively easier, since generally the voltage will be maintained within 10% or so of a preset magnitude such as 110 volts. Thus a conventional A-to-D converter of a relatively limited bit resolution, such as 10 or 12 bits, will be capable of providing adequate resolution for measuring the voltage component.
It is a principal object of this invention to provide apparatus and techniques capable of making highly accurate electric power measurements in a digital signal format at relatively modest cost.